CVE-2026-28759: Mattermost Server

Medium severity, CVSS 4.3. EPSS: 0.3% chance of exploitation in the next 30 days.

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.14 (fixed in 10.11.14); from 11.4.0, before 11.4.4 (fixed in 11.4.4); from 11.5.0, before 11.5.2 (fixed in 11.5.2)

Published 2026-05-18. Last modified 2026-06-17.